Yusuf Hassan, spokesman for the UN High Commissioner for Refugees, said nearly one million refugees could arrive in Pakistan and nearly half a million in Iran. Plans were underway in Tajikistan, Uzbekistan and Turkmenistan to accommodate a further 150,000.

"The UNHCR is planning for an expected 1.5 million Afghans fleeing into neighbouring countries," said Mr Hassan.

The agency said that tens of thousands of Afghans were on the move inside the country to try to avoid threatened strikes against Osama bin Laden and his Taliban protectors.

Pakistan is planning 100 refugee camps in North West Frontier Province, near the Afghan border, for the tide of people expected after any American strike on Afghanistan. Yet all the locations identified are isolated, drought stricken and at risk of attack.

Relief workers fear that decisions already taken by the Pakistani government will make the situation worse. None of the camps will be ready for at least two weeks. "We've got to hope that the war doesn't start before then," said one aid worker.

All the new camps are in barren areas, with little access to water. The camps will be controlled and sealed off by the Pakistani army, effectively rendering the refugees prisoners.

Moreover, all will be located in semi-independent tribal areas like Waziristan and Mohmand. These are strongholds of Islamic fundamentalism where many of the local people are fiercely supportive of Afghanistan's Taliban regime.

They will view both refugees and western aid workers with open hostility.

Pakistan already hosts two million Afghan refugees and President Musharraf's government views any new influx as a major security problem.

Instead, new camps will be constructed in arid areas where the water table is 1,000ft beneath the surface, making the drilling of bore-holes extremely difficult. Oxfam will provide water and sanitation for the camps and its workers are preparing to face a myriad of obstacles.

Alex Renton, spokesman for Oxfam, said: "We have a lot of sympathy with the Pakistani government in the crisis they are facing and we're very relieved that they will help the refugees from Afghanistan.

"The camps are far from ideal. They are in drought stricken areas, they are extremely unsafe and it will be very difficult for us to work, but we're pulling together to get this enormous job done."

Aid workers fear that the camps will become little more than tightly guarded, temporary holding centres, from which the refugees will be returned to Afghanistan as soon as possible.